A motor vehicle steering system is configured as shown in FIG. 7 so that a rotation of a steering wheel 1 is transmitted to an input shaft 3 of a steering gear unit 2 and a pair of left and right tie rods 4, 4 are pushed or pulled in association with the rotation of the input shaft 3 to impart a steering angle to front road wheels. The steering wheel 1 is supported and fixed to a rear end portion of a steering shaft 5. This steering shaft 5 is supported rotatably in a cylindrical steering column 6 in such a state that the steering shaft 5 is inserted through the steering column 6 in an axial direction. Additionally, a front end portion of the steering shaft 5 connects to a rear end portion of an intermediate shaft 8 via a universal joint 7, and a front end portion of the intermediate shaft 8 connects to the input shaft 3 via a different universal joint 9. It is noted that in the illustrated example, an electric power steering system is incorporated. Because of this, an electric motor 10, which constitutes a power source for supplying an assisting force, is provided at a front end portion of the steering column 6 by allowing the motor 10 to be supported on a housing 11 which is fixed to the front end portion of the steering column 6. Then, an output torque (an assisting force) of this electric motor 10 is imparted to the steering shaft 5 via a gear unit provided within the housing 11.
In general, in the steering system described above, a tilt mechanism is incorporated which adjusts the height position of the steering wheel 1 according the build and driving posture of a driver. As a steering system having the tilt mechanism described above, there are widely known constructions which are described, for example, in Patent Documents 1 to 3. Although the afore-said conventional construction depicted in FIG. 7 is like the constructions described in Patent Document 2 to 3, in the case of this conventional construction, an upper front end portion of the housing 11 is supported so as to oscillate to be displaced relative to a vehicle body 12 by a tilt shaft 13 which is disposed in a widthwise direction (a widthwise direction of the vehicle body on which the steering system is placed). Additionally, a supporting bracket 14 is placed at a portion close to a rear end of an axial intermediate portion (an area excluding both ends which differs from a central portion and which includes portions lying near end portions) of the steering column 6, and this supporting bracket 14 is supported on the vehicle body 12. This supporting bracket 14 includes a pair of left and right supporting plate portions 15 which are spaced apart in the widthwise direction and is supported on the vehicle body 12 in such a state that the portion lying close to the rear end of the axial intermediate portion of the steering column 6 is held by both the supporting plate portions 15 from both sides in the widthwise direction. In addition, a displacement bracket 16 is provided at a portion lying on a lower surface of the axial intermediate portion of the steering column 6 and held by both the supporting plate portions 15.
Additionally, vertically elongated holes 17 which have an arc-like shape centered at the tilt shaft 13 and which are long in a vertical direction are formed individually in both the supporting plate portions 15, and through holes 18 are formed in portions of the displacement bracket 16 which are aligned partially with the vertically elongated holes 17. Additionally, the construction shown in FIG. 7 incorporates a tilt and telescopic mechanism which tilt and telescopic steering system which can adjust a back-and-force position as well as a height position of the steering wheel 1, and therefore, the through holes 18 are made into elongated holes which are long in the axial direction of the steering shaft 5 and the steering column 6. The steering shaft 5 and the steering column 6 are constructed to extend and contract in association with the provision of the elongated holes 18. A more specific construction of the tilt steering system which incorporates the telescopic mechanism will be described by reference to FIGS. 8 to 9 which are disclosed in Japanese Patent Application No. 2011-214698.
The steering column 6 can extend and contract its whole length by allowing a rear portion of an inner column 20 which is disposed on a front side of the steering column 6 to slidably fit in a front portion of an outer column 19 which is disposed on a rear side thereof. Of the outer and inner columns, a slit 21 is provided in the front portion of the outer column 19 which is die-cast of, for example, a light alloy so that a bore diameter of the front portion can elastically be expanded and contracted. Additionally, a pair of left and right held plate portions 22, 22 are provided at portions which hold the slit 21 from left- and right-hand sides thereof, so that the displacement bracket 16 is made up of these held plate portions 22, 22. The through holes 18, 18 which are elongated in the longitudinal direction are formed individually in the held plate portions 22, 22. Additionally, the vertically elongated holes 17, 17 which have the partially arc-like shape centered at the tilt shaft 13 (refer to FIG. 7) and which are long in the vertical direction are formed in the pair of left and right supporting plate portions 15, 15 which are provided on the supporting bracket 14 so as to be disposed at the portions which hold the displacement bracket 16 from the left- and right-hand sides thereof. Then, a tilt rod member 23 such as a tilt bolt, a push-pull rod or the like is inserted through both the vertically elongated holes 17, 17 and both the through holes 18, 18 in the widthwise direction.
Further, an adjustment lever 24 (a tilt lever), an anchor portion 25 and a cam device 26 are provided on the tilt rod member 23 at an axial end portion, at the other axial end portion and at a portion lying close to one end of an axial intermediate portion, respectively, to thereby make up a tilt lock mechanism which expands and contracts a space defined between inner surfaces of the supporting plate portions 15, 15 based on the oscillation of the adjustment lever 24. The anchor portion 25 is something like a head portion of a bolt which is provided at the other end portion of the tilt rod member 23 and has a circular disk-like shape as a whole. Then, a first engagement projecting portion formed on an inner surface of the anchor portion 25 is brought into engagement with the vertically elongated hole 17 (the right one in FIG. 9) which is one of the vertically elongated holes 17 so as to be displaced only along the vertically elongated hole 17. Consequently, the tile rod member 23 can move up and down along both the vertically elongated holes 17, 17 but never rotates about an axis thereof.
Additionally, the cam device 26 is made up of a combination of a drive cam 28 and a driven cam 29 (a presser member). These cams 28, 29 have center holes through which the tilt rod member 23 is inserted and are formed into a circular ring shape as a whole. In addition, a drive-side cam surface and a driven-side cam surface, which are a circumferential recess and a circumferential projection, respectively, are formed on surfaces of the cams 28, 29 which face each other. Additionally, a second engagement projecting portion 30 which is formed on an inner surface (a side surface lying centrally with respect to the widthwise direction of the steering column 6) of the driven cam 29 is brought into engagement with the vertically elongated hole 17 (the left one in FIG. 9) which is the other of the vertically elongated holes 17 so as to be displaced only along the vertically elongated hole 17. Consequently, the driven cam 29 can also move up and down along the vertically elongated hole 17 but never rotates about an axis thereof. Further, the drive cam 28 is connected to be fixed to a proximal end portion of the adjustment lever 24 so as to rotate backwards and forwards around a circumference of the tilt rod member 23 as the adjustment lever 24 oscillates backwards and forwards. Additionally, a thrust bearing 32 is provided between a nut 31 which is securely screwed on the other end portion of the tilt rod member 23 and an outer surface (a side surface lying outboard with respect to the widthwise direction of the steering column 6) of the proximal end portion of the adjustment lever 24 so as to enable the drive cam 28 to oscillate backwards and forwards while bearing a thrust load exerted on the drive cam 28. The nut 31 is crimped or the like so as to be prevented from being loosened.
When adjusting the position of the steering wheel 1, an axial dimension of the cam device 26 is contracted by oscillating the adjustment lever 24 in a predetermined direction (in general, in a downward direction), so as to expand a space defined between the driven cam 29 and the anchor portion 25. As a result of this, a surface contact pressure at abutment portions between the inner surfaces of the supporting plate portions and outer surfaces of the held plate portions 22, 22 is reduced or lost, and at the same time, the bore diameter of the front end portion of the outer column 19 is elastically expanded, whereby a surface contact pressure at an abutment portion between an inner circumferential surface of a front end portion of the outer column 19 and an outer circumferential surface of a rear end portion of the inner column 20 is reduced. In this state, a vertical position and a longitudinal position of the steering wheel 1 can be adjusted within ranges where the tilt rod member 19 is allowed to move within the vertically elongated holes 17, 17 and the through holes 18, 18. After the steering wheel 1 is moved to a desired position, the adjustment lever 24 is caused to oscillate in a reverse direction (in general, in an upward direction) to expand the axial dimension of the cam device 26 to thereby contract a space defined between the inner surfaces of the supporting plate portions 15, 15. This increases the surface contact pressure at the abutment portions between the inner surfaces of the supporting plate portions 15, 15 and the outer surfaces of the held plate portions 22, 22, and at the same time, the bore diameter of the front end portion of the outer column 19 is contracted elastically to increase the surface contact pressure at the abutment portion between the inner circumferential surface of the front end portion of the outer column 19 and the outer circumferential surface of the rear end portion of the inner column 20, whereby the steering wheel 1 can be held in the adjusted position.
Various mechanisms which can adjust the position of a steering wheel and hold the steering wheel in an adjusted position by adjusting a force which presses against outer surfaces of a pair supporting plate portions which make up a supporting bracket are known in addition to the construction shown in FIGS. 8 to 9. For example, a construction is also known in which a tilt rod member is allowed to rotate about an axis thereof. When this construction is adopted based on the construction shown in FIGS. 8 to 9, a thrust bearing is provided between the anchor portion 25 and an outer surface of one of the supporting plate portions 15, and the proximal end portion of the adjustment lever 24 is connected to be fixed to the other end portion of the tilt rod member 23. Further, a construction similar to the construction shown in FIGS. 8 to 9 is also known in which a first engagement projecting portion 27 which is provided on an inner surface of a anchor portion 25 is brought into engagement with one of vertically elongated holes 17 so as to be displaced only along this vertically elongated hole 17, and a nut 31 (a presser member) which is securely screwed on the other end portion of a tilt rod member 23 is connected to be fixed to a proximal end portion of an adjustment lever 24. In the case of these constructions, the nut 31 is rotated by the adjustment lever 24 so as to expand or contract a space defined between the nut 31 and the anchor portion 25. In the case of either of the constructions, the engagement projecting portion needs to be brought into engagement with the vertically elongated hole so as to be displaced only along the vertically elongated hole with the tilt rod member prevented from rotating about the axis thereof at least at one location.
In the case of either of the constructions, to ensure a sufficient force to hold the steering wheel in the adjusted position, the surface contact pressure at the abutment portions between the inner surfaces of the supporting plate portions 15, 15 and the outer surfaces of the displacement bracket 16 needs to be ensured in such a state that the space defined between the anchor portion 25 and the presser portion such as the driven cam 29 is contracted by operating the adjustment lever 24. Then, in order to ensure the surface contact pressure at both the abutment portions, a space between portions of the supporting plate portions 15, 15 which hold the displace bracket 16 therebetween needs to be contracted in an ensured fashion. As shown in FIGS. 7 to 9, in the case of the construction in which the displacement bracket 16 is provided on a lower side of the steering column 6, it is relatively easy to ensure the surface contact pressure at the abutment portions. This is because even when the steering wheel 1 is positioned at an upper end of an adjustment range thereof, the portions of the supporting plate portions 15, 15 which hold the displacement bracket 16 therebetween are spaced away from upper ends of the supporting plate portions 15, 15 and the rigidity against effort to move those portions towards each other is suppressed to a low level.
In contrast to this, as described in, for example, Patent Document 4, in a construction in which a displacement bracket is provided on an upper side of a steering column, when a steering wheel is positioned at an upper end of an adjustment range thereof, it becomes difficult to ensure a surface contact pressure at abutment portions between inner surfaces of supporting plate portions and outer surfaces of the displacement bracket. Namely, as depicted in FIGS. 1 to 2 which show an embodiment of the invention, a construction is considered in which a displacement bracket 16a is provided on an upper side of an outer column 19 which makes up a steering column 6a. This construction is advantageous in that the degree of freedom in designing a protection for knees of a driver when a vehicle is involved in a collision is enhanced by such an extent that the displacement bracket 16a does not project downwards of the steering column.
However, when the displacement bracket 16a is provided on the upper side of the outer column 19a as described above, the displacement bracket 16a is situated near upper end portions (proximal end portions) of a pair supporting plate portions 15a, 15b which make up a supporting bracket 14a. When a space defined between an anchor portion 25 and a driven-side cam 29 (refer to FIG. 9 for the driven cam 29) of a cam device 26 is contracted by operating an adjustment lever 24a, the supporting plate portions 15a, 15b are displaced about the upper end portions thereof. The rigidities of the supporting plate portions 15a, 15b become higher as they extend towards the upper end portions thereof. Therefore, in the construction shown in FIGS. 1 to 2, when attempting to ensure a surface contact pressure at both the abutment portions with a steering wheel positioned at an upper end of an adjustment range thereof, a large operating force needs to be exerted on the adjustment lever 24a. In this way, the fact that a large difference in operating force of the adjustment lever 24a is produced depending upon whether the steering wheel 1 is positioned in an upper or lower position is not preferable since a driver who operates the adjustment lever 24a is caused to feel a sensation of physical disorder.
The sensation of physical disorder described above can, of course, be mitigated by adopting a configuration in which a height dimension of the supporting plate portions 15a, 15b is increased so as to ensure a distance between the displacement bracket 16a and the upper end portions of the supporting plate portions 15a, 15 even with the steering wheel positioned at the upper end of the adjustment range thereof. However, this means cannot be adopted in many cases excluding a case where extra space is available for placement of a steering system since a position where the steering column 6a is placed is situated on a lower side as long as a height position of a portion provided on the side of a vehicle body where the supporting bracket 14a is assembled remains the same.
In the case of the construction described in Patent Document 4, upper half portions of inner surfaces of the anchor portion and the driven-side cam are depressed further than lower half portions, or peripheral portions of upper end portions of vertically elongated holes on outer surfaces of a pair of supporting plate portions are depressed. However, in the case of the former construction, since a direction in which the anchor portion and the driven-side cam are assembled is limited (they cannot be assembled in a vertically reverse direction), the efficiency of assembling work is reduced. In addition, in the case of the latter construction, not only is an extra step of depressing the outer surfaces of the supporting plate portions through surface pressing necessary but also a step is necessary of removing burrs which are produced in association with the surface pressing, resulting in an increased production cost. Additionally, in either of the constructions, it is not possible to make a fine adjustment of a force necessary to operate the adjustment lever, and hence, the difference in operation force to operate the adjustment lever which is caused in association with the difference in vertical position of the steering wheel cannot necessarily be eliminated sufficiently. Further, in the case of either of the constructions, abutment areas between the inner surfaces of the anchor portion and the driven-side cam and the outer surfaces of the supporting plate portions are narrowed to increase the surface contact pressure at the abutment portions, and therefore, these inner surfaces and outer surfaces are plastically deformed easily. Then, when they are plastically deformed, it is not possible to effect a smooth adjustment of the vertical position of the steering wheel.